Perpendicular magnetic recording systems have been developed for use in computer hard disc drives to provide higher liner density than longitudinal recording. FIG. 1, obtained from Magnetic Disk Drive Technology by Kanu G. Ashar, 322 (1997), shows magnetic bits and transitions in longitudinal and perpendicular recording. In a longitudinal recording there exists a demagnetization field between two magnetic bits. These demagnetization fields tend to separate bits, making transition space between bits, that is, transition parameter a, large as shown in FIG. 1 (a). At very high bit densities, the limiting parameter may be the length of the transition region. Perpendicular recording bits do not face each other, and hence they can be written at closed distances as shown in FIG. 1 (b).
A typical perpendicular recording head includes a trailing write pole, a leading return or opposing pole magnetically coupled to the write pole, and an electrically conductive magnetizing coil surrounding the yoke of the write pole as shown in FIG. 2, Magnetic Disk Drive Technology by Kanu G. Ashar, 323 (1997). The ring-type head shown in FIG. 2 is generally not used for perpendicular recording anymore. The writer portion of the head is still being used, but the reader portion is not. Perpendicular recording media may include magnetic media and an underlayer as shown in FIG. 2. The magnetic media could be a hard magnetic recording layer with vertically oriented magnetic moment and the underlayer could be a soft magnetic underlayer to enhance the recording head fields and provide a flux path from the trailing write pole to the leading or opposing pole of the writer. The magnetic flux passes from the write pole tip, through the hard magnetic recording track, into the soft magnetic underlayer, and across to the opposing pole. Such perpendicular recording media may also include a thin interlayer between the hard recording layer and the soft magnetic underlayer to prevent exchange coupling between the hard and soft layers. The soft magnetic underlayer helps also during the read operation. During the read back process, the soft magnetic underlayer produces the image of magnetic charges in the magnetically hard layer, effectively increasing the magnetic flux coming from the medium. This provides a higher playback signal.
The soft magnetic underlayer is located below a recording layer and alters the flux path from the recording head main pole to the return pole. For a thick, high permeability SUL, the altered flux path is similar to that which would result from placing a mirror image of the recording head below the SUL surface. Thus, the net recording field at the hard magnetic recording layer becomes fairly large compared to the field generated in the case of longitudinal recording. Magnetic flux flows from head through the SUL to return pole crossing twice through the recording layer. The quality of the image, and therefore the effectiveness of the soft magnetic underlayer, depends upon the permeability of the soft magnetic underlayer.
Conventional perpendicular magnetic recording media comprise magnetic SUL, seed layers, HCP (Hexagonal Close Packed)-structured interlayers, Co-based magnetic recording layers, and carbon overcoat. The SUL has significant effect on the crystallographic orientations of the recording layers. Some typical materials used as SUL such as FeCo30.5B12.8, feature high saturation induction, Bs, moderate magnetic permeability, μ, and provide an appropriate template for the seed layers, interlayers, and recording layers to grow on. The magnetic performance and crystallographic structure of the media with such SUL are generally reasonable. However, media yields often suffer due to a higher incidence of target spitting. Also, in order to suppress stripe domain formation in SULs featuring high magnetostriction constant, including FeCoB, a laminated SUL structure with spacer layers, such as Ta, could be interposed between critically thin soft layers. The medium is sputter-deposited under room temperature, which is the typical process temperature for granular recording media. Each spacer layer needs one deposition chamber and requires an additional deposition step. It is therefore advantageous to have a perpendicular recording medium without a spacer layer between SUL laminations or to eliminate the need for lamination by the use of appropriate SUL materials that suppress stripe domain.